1. Field of the Invention
The present invention relates to a motor driving device and to a control method for the motor driving device, and specifically to a motor driving device for a brushless direct-current (hereinafter, referred to as “DC”) motor and to a control method for the motor driving device.
2. Description of Related Art
Currently, in the field of consumer products, such as washing machines, refrigerators and air conditioners, for which machine downsizing has been demanded, small-sized and high-power permanent magnet synchronous motors have broadly been employed.
Also, in recent years, progress in technical innovation of power devices such as a Metal-Oxide-Semiconductor Field-Effect Transistor (hereinafter, referred to as “MOS FET”) has been seen. Thus, it has become possible to perform inverter control in which a commercial alternating current (hereinafter, referred to as “AC”) power supply is first rectified to be converted into a DC, and then re-produced to have a desired drive waveform by the switching-on/off of power devices. This inverter control easily provides power consumption reduction and also provides easy control. Currently, brushless DC motors, in which such a permanent magnet synchronous motor as described above is driven by inverter control, have widely been used.
FIG. 4 illustrates an inverter circuit 1 in a commonly-used brushless DC motor. As illustrated in FIG. 4, the inverter circuit 1 includes transistors Q1 to Q6. The transistors Q1 and Q2, Q3 and Q4, and Q5 and Q6 are respectively connected in series between a DC power supply voltage VDD and a ground voltage GND. Control signals U+, U−, V+, V−, W+ and W− are input to the transistors Q1 to Q6, respectively.
FIG. 5 illustrates an example of operation waveforms of the voltages of these control signals U+, U−, V+, V−, W+ and W−. Based on the pulse waveforms illustrated in FIG. 5, the transistors Q1 to Q6 perform switching operation in which they are repeatedly turned on or off. For example, at times t0 to t2, the control signals U+ and V− are coincidentally at a high level, and thus, the transistors Q1 and Q4 are coincidentally in an on-state. Consequently, currents flow in the coils in the U-phase and the V-phase of a brushless DC motor 2. Similarly, at times t2 to t4, control signals V+ and W− are coincidentally at a high level, and thus, the transistors Q3 and Q6 are coincidentally in an on-state. Consequently, currents flow in the coils in the V-phase and the W-phase of the brushless DC motor 2. Subsequently, the transistors are switched on/off based on the control signals in such a manner as described above, enabling the inverter circuit 1 to generate drive current for the brushless DC motor 2.
In this example, the brushless DC motor 2 is a three-phase motor. Accordingly, the on/off state of the transistors Q1 to Q6 is adjusted so that currents flowing in the coils in the U-phase, the V-phase and the W-phase of the brushless DC motor 2 flow in phases shifted from one another by 120°. The control signals U−, V− and W− are signals that are inversions of the control signals U+, W+and V+, respectively.
Furthermore, pulse width modulation (hereinafter, referred to as “PWM”) is used for motor drive control by the switching mentioned above. This PWM control is currently most commonly used as a DC motor control method. A brief description of PWM control will be provided with reference to FIGS. 6A and 6B. The graph in FIG. 6B illustrates one of the control signals U+, U−, V+, V−, W+ and W− being subjected to pulse-width modulation, for example, the control signal U+. Each of the other control signals is a signal having a wavelength similar to that of the example or a signal that is an inversion thereof, though its phase is shifted from that of the example.
PWM control in the example, as illustrated in FIG. 6A, uses a triangle wave as a carrier. Also, in order to control, e.g., the rotation speed of the motor to have a desired value, a command voltage signal, which is illustrated in FIG. 6A, is used. This command voltage signal and the triangle wave are compared with each other to determine the pulse widths of the control signal U+, as illustrated in FIG. 6B.
As illustrated in FIG. 6B, where the amplitude voltage of the command voltage signal is high, the widths of the pulses of the control signal U+ are large. Conversely, where the amplitude voltage is low, the pulse widths of the control signal U+ are small. Where the pulse widths are large, the on-state of the transistor lasts for a relatively long time, resulting in an increase in the currents flowing in the coils of the motor, and thereby raising the rotation speed of the motor. Conversely, where the pulse widths are small, the on-state of the transistor lasts only for a short time, thereby lowering the rotation speed of the motor. As described above, in PWM control, a command voltage signal is subjected to pulse width modulation, and the rotation speed of the motor is controlled by, e.g., the control signal U+ subjected to pulse width modulation.
Here, in inverter control for a brushless DC motor as described above, where it becomes unable to perform motor drive control due to, e.g., sudden deceleration of the motor or a system failure, the motor enters a regeneration (power generation) state due to the load-side inertia, generating a large back electromotive force (emf). In order to prevent the motor, the transistors, etc., in the inverter circuit, or a smoothing capacitor in a converter circuit that supplies the inverter circuit with power, from being broken due to such back electromotive force, a mechanism for back electromotive force removal is needed.
JP-A-HEI-6-343291 discloses a back electromotive force removal device 3 that upon the voltage on the input side of an inverter being abnormally increased by a back electromotive force from a motor, a current is made to flow in a regeneration load resistor to remove the back electromotive force. FIG. 7 illustrates a configuration of the back electromotive force removal device 3. As illustrated in FIG. 7, the back electromotive force removal device 3 includes a power supply unit 4, a back electromotive force detection unit 5, a MOS FET base driver 6, a back electromotive force removal unit 7, a first display unit 8 and a second display unit 9.